Hall effect in carbon nanotube thin films

We have investigated Hall coefficient and magnetoresistance in thin films of single-walled carbon nanotubes, prepared in four different ways. Hall voltages are linear for all samples in magnetic fields. up to 6 T, and the measured carrier density lies in similar to 10(21)-10(22) cm(-3). Whereas earlier Hall-effect experiments reported similar to 10(18)-10(19) cm(-3) for the carrier density, our results are consistent with the theoretically predicted value of similar to 10(22) cm(-3), calculated for the aligned metallic CNTs. The signs of the Hall coefficients are positive in general, indicating that majority carriers are holes in these films. In a nanotube film with the lowest conductivity, however, we find the Hall coefficient reverses the sign at low temperature around T = 15K. The origin of the sign change is not clear. In strongly localized regime, the Hall effect can be anomalous. (C) 2014 Elsevier B.V. All rights reserved.